The River Thames has been integral to the prosperity of London since Roman times. Explorers sailed away on voyages of discovery to distant lands. Colonies were established and a great empire grew. Funding their ships and cargoes helped make the City of London into the world's leading financial centre. In the 19th century a vast network of docks was created for ever-larger ships, behind high, prison-like walls that kept them secret from all those who did not toil within. Sail made way for steam as goods were dispatched to every corner of the world. In the 19th century London was the world's greatest port city. In the Second World War the Port of London became Hitler's prime target. It paid a heavy price but soon recovered. Yet by the end of the 20th century the docks had been transformed into Docklands, a new financial centre.